Bug 32828: Fix cataloguing/value_builder/unimarc_field_115a.pl

Bug 32828: Fix cataloguing/value_builder/unimarc_field_115a.pl

Use event parameter

This is part of the refactoring happening in bug 30975

Test plan:
1. Verify that the plugin continues to work as before

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 0117e26d9e099256567d2e97bbdb5ec7b2f61e77)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
(cherry picked from commit eaac1f9659bb7ff904f078014be8a56ce39f9752)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>